The invention relates to a process for determining the location and/or the size of rock cavities, particularly artificially generated rock cracks.
A process for determining the location or the extent or rock cavities or formations which is based on electrical resistance measurement is known. To exactly determine the presence of small cavities by this method, it is necessary to perform several drillings and to introduce resistance measuring probes. If the cavities or rock cracks are artificially generated, this prior art process largely fails, particularly because the resistance changes evoked by the artificial cracks cannot be exactly interpreted. This prior art method of measurement suffers also from the fact that it is dependent, not so much on the actual cavity or the inclusion of various material in the cavity, but more importantly on the moisture content of the rock.
It is also theoretically possible to utilize acoustic measuring processes to determine the location and the course of cracks. Thermal tensions and pressure variations in the vicinity of a crack, particularly in coal and country rock (Nebengestein) generate acoustic signals which can be measured. These signals, however, are not locally bounded and are indefinite, both with respect to their nature and their duration, so that an exact interpretation of the signals picked up by the geophones is not possible.
In the particular case of the artificial production of rock cracks, mostly designated as Frac technology, it is essential that the size, position and course of the cracks be determined. All of the known measuring processes for the determination of artificial cracks have the common disadvantage that, because of limited penetration depth and range, a satisfactory registration and adequate resolution and interpretation of the measurement signals is impossible.
The determination of the exact position and extent of rock cracks is of extreme importance in processes for the recovery of gas from coal, wherein coal seams are opened up by drillings, between which opened seams, by reason of natural permeability in the formation, there is present a permeable connection through naturally occurring of artificial gaps or crack fission systems. Into one of the bores of the drill, particularly an injection bore, gases or vapors having as the main components thereof air, water vapor, oxygen or mixtures of these, are introduced which penetrate into a seam and support an initial combustion process and a subsequent gasification process. The resulting gas mixture is then removed from the production bore.
As a rule, the natural permeability is not satisfactory. As a result, the permeability of the seam must be artifically reinforced or even established. For example, rock cracks can be generated hydraulically by the so-called "hydraulic fracturing" method. In this method the energy of the frac fluid is used to induce cracking in the formation and, simultaneously, through the transport capacity of the fluid a supporting agent is introduced into the newly created crack, to hold the crack open against the rock pressure.
Both the cavity formed from the artificially generated crack and the cavity arising from the combustion reaction are, with the conventional methods described, virtually incapable of having their position and extent determined. In particular, it is also impossible, in practice, to monitor the extent, the location and the progress of the combustion reaction.
Accordingly, underlying the present invention is the problem of providing a process for determining the location and/or the extent of rock cavities, especially of artificially generated rock cracks, which is exact, simple and dependable and which produces measurable results which are interpretable.